Why is John F Kennedy considered one of the greatest Presidents? what did he do for the US well as the world - please give examples . ( high school level )

Kennedy's accomplishments were rather limited, but this is mainly due to his term being cut short by an assassination.  Kennedy was open to dialogue on civil rights, though he proceeded quite cautiously in order to avoid upsetting Southern Democrats in Congress.  Kennedy's rhetoric in his first inaugural address--"ask not what your country can do for you; ask what you can do for your country," has inspired generations.  Kennedy's establishment of the Peace Corps is a lasting legacy of his administration, and the organization has helped millions in the developing world.  Kennedy also urged Americans to put a man on the moon before 1970; while he would not live to see it, the lunar landing in 1969 was part of his legacy.  
Kennedy's greatest foreign policy achievement was the Cuban Missile Crisis in which he was able to defuse a possible nuclear conflict with the Soviet Union over nuclear bases in Cuba.  He opened direct negotiations with the Kremlin and removed missile bases from Turkey which threatened the Soviet Union.  This proved to be a minor but important thaw in American-Soviet relations in the early 1960s.